The present invention relates to air monitors. Specifically, the present invention relates to on-line air monitors unaffected by the absence or presence of radon progeny.
Continuous air monitors are commonly used in radioactive environments. In this environment, a continuous air monitor (CAM) is located downstream of a high efficiency particulate air (HEPA) filter. When operating properly, the HEPA filter traps all of the radioactive particles therein and the CAM should detect only a minimum of radioactive particles.
However, when not operating properly, the HEPA filter may not trap all of the radioactive particles therein and the CAM will detect the radioactive particles and signal the failure of the HEPA filter.
Conventional air monitors use borosilicate glass filters. One example of the use of borosilicate glass filters is U.S. Pat. No. 4,426,214 to Vandrish. The patent discloses an air sampling device using a glass filter assembly. Plastic layers sandwich the glass fiber filter therebetween. The filter assembly is inserted into the air sampling device for monitoring radioactive particles in the air.
Borosilicate glass filters have numerous drawbacks. First, the glass filters are easily damaged during use. The glass filter can easily tear during assembly or operation. Second, the glass filter can absorb moisture and weaken. Third, the sampling device must be operated at a relatively low vacuum pressure to avoid damaging the filter. Finally, the glass filters are not reusable. Borosilicate glass filters are subjected to, for example, an acid dissolving process to determine alpha accumulation.
Our environment includes distributed particles of Radium 226. The half-life of Radium 226, 1602 years, ensures a constant release of Radon 222, its decay product. Radon 222 is a noble gas. The half-life of Radon 222 is 3.82 days. Radon 222 continuously decays into Bismouth 214 and Lead 214. Bismouth 214 and Lead 214 have half-lives of approximately 30 minutes. Bismouth 214 is a gamma emitter. Lead 214 is a beta emitter. Alpha radiation is also present during the decay of radon. Alpha radiation is present due to the presence of Polonium 218 and Polonium 214 in the decay chain.
Weather conditions influence the amount of Radon release. Fog, rain and temperature inversions, for example, increase the amount of Radon release.
Another drawback of conventional air monitors is their inability to differentiate between a naturally-occurring release of Radon progeny and a man-made release (i.e. HEPA filter failure). During certain weather conditions, the increased rate of Radon progeny release causes false alarms in conventional air monitors. Frequent false alarms can desensitize individuals. Rather than immediately responding to an alarm, an individual can become complacent, believing every alarm to be a false alarm.
One attempt to reduce false alarms involves highly complex electronics. EG&G Ortec Alpha/Beta Continuous Air Monitor model number OS3300 is such a device. The complex nature of the circuitry is inherently undesirable. In addition, the success of the device is also suspect.
Clearly, there is room for improvement in the art.